libldap poll(2) use

I've added and enabled rough but working (on my system)
libldap code which uses poll(2) instead of select(2).
This is intended to allow clients to use very large
descriptor values. It doesn't increase the number
of descriptors that might be used in managing an
LDAP handle, which is still limited to FD_SETSIZE.
That is, one can FD_SETSIZE connections in each LDAP
handle, any of which can be on any valid descriptor
value.
As I said above, the code is a bit rough... if you
see problems, please report them through normal
channels.
Kurt